I was flipping through the Xi'an dialect volume of the Great Dictionary of Modern Chinese Dialects.
On p. 206 I came across:
and on p. 207 there is:
There is the /pf/ initial and the aspirated consonant /pf‘/.
I'm quite curious to see what these sound like, especially since /pf/ seems like a topolectical equivalent of zh in MSM and /pf‘/ one of ch.
What do the /pf/ & /pfʰ/ initials sound like in Central Plains Mandarin?
I live near Xi'an and I can confirm /pf/ & /pfʰ/ are accurate. It's literally p + f. It's like pronouncing /p/ while having your teeth on the lips for /f/, so when your mouth opens, it's a natural /f/.
edit: I think it should be mentioned this is Xi'an-only. Xi'an dialect belongs to 中原官话关中片 (Central Plains Mandarin in the region of Guanzhong), but other dialects in the same subcategory doesn't have /pf/, let alone the broader category Central Plains Mandarin.
"pfæ" 西安and one result came up with 汉语方言大辞典（？）.pdf. Interesting information in there; one point is that in the traditional speech in that area, 合口呼 syllables (rimes beginning with ㄨ) are not compatible with initials like 磚 and 穿. [b, v, pf, ...] are some adaptations in those topolects. Their 水 is apparently romanised as "fei".